The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen", which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort for advancing a retorting zone through the fragmented mass. Kerogen in oil shale in the retorting zone is decomposed to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing retort inlet mixture into the retort as a gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the gaseous combustion zone feed into the combustion zone, the combustion zone is advanced through the retort.
Gas from the combustion zone passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
As used herein, the term "processing gas" is used to indicate gas which serves to advance a processing zone such as a combustion zone, a retorting zone, or both a retorting zone and combustion zone, through the fragmented mass in an in situ oil shale retort, and includes, but is not limited to, an oxygen supplying gas introduced into a retort for advancing a combustion zone and retorting zone through a retort and a hot retorting gas which can be introduced into a retort or generated in a combustion zone in a retort for advancing a retorting zone through a retort.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. A portion of the liquid products collected at the bottom of the retort can result from condensation of hydrocarbon vapors from the retorting zone on the cooler particles in the fragmented mass on the advancing side of the retorting zone. Water vapor from water produced in or added to the retort can also condense on such cooler particles. An effluent gas, referred to herein as off gas, containing combustion gas generated in the combustion zone, gaseous products including methane produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process is also withdrawn from the bottom of the retort.
The products recovered from the bottom of the retort include off gas, liquid shale oil, and water. Some separate shale oil and water phases can be recovered at the bottom of the retort but much of the product is in the form of a viscous emulsion of water and shale oil. The troublesome existence of stable shale oil-water emulsions has been recognized; see, for example, U.S. Pat. No. 3,929,625 to Lucas, which describes a process employing chemical surface active agents and water for removing suspended solids from shale oil.
Before such emulsion is broken, it can be a thixotropic fluid that flows so long as pumping is continued. When pumping is stopped such fluid tends to set up much like a gel and it is difficult to get moving again. The emulsion can be predominantly in the form of an oil-in-water-in-oil emulsion in which a continuous shale oil phase contains dispersed water droplets which in turn contain dispersed shale oil droplets. Such emulsion has proven to be highly resistant to petroleum de-emulsifying additives. This emulsion forms in the retort under conditions that are not fully understood and may very well involve condensation of water vapor, or co-condensation of water vapor and hydrocarbon vapors, on cooler particles in the fragmented mass on the advancing side of the retorting zone. It is known that the emulsion from the bottom of the in situ oil shale retort is particularly difficult to break by known techniques. A variety of chemical treatments of emulsion have been attempted to cause a separation of the shale oil and water into separable phases and no fully satisfactory economical technique has been discovered.
The large amount of water in emulsion with the shale oil significantly affects its properties, including its viscosity. The emulsion is sluggish and difficult to handle and can involve a substantial storage and shipment problem since there may be up to 75% water in the emulsion. It is also desirable to separate water from the shale oil for use of the water at the site of retorting. It is therefore desirable to provide a technique for economically breaking the shale oil-water emulsion from an in situ oil shale retort.
Petroleum-water emulsions are sometimes encountered in producing petroleum from wells. Emulsion breaking chemicals can be used for separating such an emulsion. A heater-treater can be used either alone or in combination with chemicals. A heater-treater is essentially a large vessel wherein an emulsion is heated by immersed heater tubes and thereafter travels over trays or through a filtering medium to separate petroleum and water. Average residence time of petroleum in a heater-treater is in the order of two to ten hours, although four to five hours seems to be most typical.
A heater-treater is shown in the Petroleum Production Handbook, edited by Thomas C. Frick and published by Society of Petroleum Engineers of A.I.M.E., Dallas, Texas (1962), along with related information at pages 6-27 to 6-35. Although this discussion concerns preparation of power oil for use in downhole hydraulic pumps for pumping oil wells, the description of the heater-treater is not unique to this application of the equipment. Details of the internal construction of emulsion heater-treaters employed in the petroleum industry are shown in U.S. Pat. Nos. 2,832,431, and 3,029,580.
Heater-treaters are also described in Petroleum Production Engineering by Lester Charles Uren, published by McGraw-Hill Book Company, Inc., New York (1962), at pages 572-575. In one such heater-treater, emulsion of petroleum and water is introduced into a layer of water maintained at 150.degree. to 200.degree. F. with immersed steam coils. Emulsion is heated by contact with the hot water and breaks down. Direct contact between the coils and the petroleum is avoided to prevent formation of a solid hydrocarbon coating on the coils. In another form of heater-treater, emulsion flows over heater tubes, where most water drops out, and into a quiet section where heated petroleum and emulsion are stored for a time while further separation of water and petroleum occurs.
Efforts have been made to separate the shale oil-water emulsion from in situ oil shale retorting using a conventional petroleum heater-treater. Heating has been in the range of from about 150.degree. to 170.degree. F. with various chemical additives and electrostatic fields employed for enhancing separation. Although some success has been obtained, the technique is not completely satisfactory and the equipment costs are high for a selected production rate.
U.S. patent application Ser. No. 737,556, filed Nov. 1, 1976, now U.S. Pat. No. 4,109,718, assigned to the assignee of the present application, and incorporated herein by this reference, describes breaking an emulsion of shale oil and water by holding the shale oil and water at a temperature above about 120.degree. F. for at least about one day. Such a process is effective but time-consuming. For high production rates, appreciable elevated temperature storage capacity can be involved. Better techniques are desirable.